The veneer is curly maple with a wood backer. I’m not sure of what type of wood the backer is. The substrate is eastern white pine. I put the vinegar on the veneer around 10 minutes before hammer veneering and didn’t notice any change. I only saw the change after starting to hammer. It felt like it was coming from the veneer hammer, but I can’t be positive.

If you are using a two ply veneer then that will be your problem. You are likely seeing the glue line between the microscopically thin curly maple and the backer. Get some raw veneer, as thick as you can find, and try again. 1/42” is about the norm these days but 1/32” is still around but harder to find. Good luck.No need for the vinegar. Hide glue cleans up with water.

Paul, I was able to use a scraper and remove the majority of the staining so I don’t believe I’m seeing through to the glue line. When I re-hammer to press bubbles down I’m seeing the markings only where I’m pressing down very hard. I then see tinted hide glue on my veneer hammer.

I’m thinking it could either be the hammer iron or finish coming off. Or maybe the 2-ply glue seeping through the wood pores? Is that possible?

Have you ever seen an iron (I think) hammer cause issues like this? Do you use brass?

I’ve used brass but my current hammers are steel and copper and no such problems. You may be getting a stain from the metal but your top photo above really looks like the factory glue (likely a urea formaldehyde) showing through. Regardless I wouldn’t consider using that stuff for this kind of woodworking (just my opinion). Raw veneer is so much better, even the thin sliced stuff we are able to get today. Some places sell limited species in 1/16”, notably Certainly Wood but there are others. For what you are doing, the thicker you can use, the better, even more so if you get into marquetry.

The discoloring is caused by reheating the two ply veneer too hot in an effort to get the veneer to relax enough to hammer down. The glue in the veneer ply is not hide glue. You cannot hammer down two ply veneer. Reheat and take it all off. If you do not want to use raw veneer(which you should IMO) get a vac. bag or use clamps and cauls. Hammer veneering with hot hide glue is easy with raw veneer plenty of video on web to get you on right track. Good luck.